A good night’s sleep isn’t just refreshing—it’s vital for engraving memories. Whether humans are trying to recall what they learned or animals are remembering where to find food, sleep plays a key role in strengthening our ability to retain and recall important information.
But how exactly does this happen? Scientists at the Institute of Science and Technology Austria (ISTA) have taken a significant step in uncovering the answer, using rats and advanced wireless technology to delve into how sleep stages optimize memory recollection.
“We found that in the early stages of sleep, rats’ brain activity reflected the memories they had just formed. Over time, this activity gradually transformed, preparing the brain to recall the learned information when the rats woke up,” explained Professor Jozsef Csicsvari, who led the research.
The study focuses on the hippocampus, a part of the brain crucial for spatial learning and navigation. This region creates a mental map of the environment, allowing animals to find their way and update their knowledge while learning.
Locations tied to rewards, like food, take on extra importance in these maps, and the hippocampus plays a key role in reinforcing these memories during sleep.
Past experiments by Csicsvari’s team showed that the more frequently a reward location is reactivated during sleep, the better the animal remembers it later. When the researchers blocked memory reactivation, the rats couldn’t recall the specific reward location.
In this new study, the researchers extended their observation period, tracking neuronal activity in the rats’ brains for up to 20 hours of sleep—far longer than previous experiments. The team discovered something unexpected: the patterns of brain activity tied to rewards underwent a process of reorganization during sleep.
“At first, the same neurons activated as during learning,” explained ISTA researcher and co-author Lars Bollmann. “But as the sleep progressed, some of these neurons stopped firing, while new ones took over. By the end, the brain activity during sleep mirrored the patterns seen when the rats woke up and remembered the rewards.”
The research also revealed that this “rewiring” process occurred during non-REM sleep, a stage known for memory consolidation. Meanwhile, REM sleep appeared to counteract this process, possibly maintaining a balance in the brain.
Why does the brain reorganize memories during sleep?
The researchers believe it’s a way to optimize memory storage.
“When we learn, memory representations are formed quickly, but they’re not always ideal for long-term storage,” Csicsvari explained. “Sleep may help refine these representations, using fewer brain resources to store a specific memory and freeing up neurons for new learning.”
Indeed, the team found that fewer neurons were involved in storing a reward memory after sleep than before. This efficiency could help the brain integrate new information into existing knowledge, ensuring strong and flexible memories.
This groundbreaking research sheds light on how sleep helps preserve memories while preparing the brain for new learning. By tracking brain activity over longer periods and linking it to memory reactivation, the ISTA team has uncovered key mechanisms explaining why a good night’s sleep is essential for humans and animals.
“Any new memories must find a way to be integrated into existing knowledge. Frequent repetitions of the new memories as well as partial change in the neuronal code may thus help optimize their integration into existing memory representations,” concludes Csicsvari.
Journal Reference
- Lars Bollmann, Peter Baracskay, Federico Stella & Jozsef Csicsvari. 2025. Sleep stages antagonistically modulate reactivation drift. Neuron. DOI: 10.1016/j.neuron.2025.02.025
Source: Tech Explorist